Mechanism of Calcite Inhibiting Scheelite Decomposition During the NaOH Decomposition Process

The method of digesting scheelite with NaOH through an autoclaving process is primarily implemented in China. However, when using this method to treat low-grade scheelite with a high calcite content, the decomposition ratio of scheelite decreases when the reaction time is prolonged. To investigate the mechanism of calcite inhibiting scheelite decomposition, the pseudoternary phase diagram of the systems at 150°C was generated. Based on the thermomechanical analysis, the authors have found an area where scheelite is stable, but calcite is unstable. In the initial stage of the sodium hydroxide decomposition of scheelite, the decomposition process is located on the right side of the area, which is unstable for scheelite and calcite, due to the high alkali concentration. As the reaction progresses, the calcite continuously decomposes and the alkali concentration in the system continues to decrease, which finally falls back to the area. This also explains the abnormal phenomenon that extending the reaction time is not conducive to scheelite leaching. In addition, thermodynamic studies show that the key to the efficient decomposition of low-grade scheelite containing calcite is by maintaining the decomposition process in the unstable area of scheelite and calcite by increasing the amount of alkali.